The invention relates to electron discharge tubes and particularly to a shielded focusing electrode assembly for a photomultiplier tube.
Photomultiplier tubes for use in severe environments, such as oil well logging, are described in my copending U.S. patent application Ser. No. 216,906, filed Dec. 16, 1980 and entitled, "PHOTOMULTIPLIER TUBE HAVING A STRESS ISOLATION CAGE ASSEMBLY". The copending patent application is assigned to the same assignee as the present invention and is incorporated herein for disclosure purposes. The photomultiplier tube described in the above cited copending patent application is known as the RCA C33016G photomultiplier tube. The tube comprises a glass envelope structure having a photocathode formed on the interior surface of the envelope faceplate. An electron multiplier including a plurality of secondary emissive dynodes and an anode is spaced from the photocathode to receive photoelectrons therefrom. An apertured shield cup is disposed intermediate the photocathode and the electron multiplier to focus the photoelectrons from the photocathode onto the electron multiplier. As described in my copending application, such tubes experience operating temperatures in the range of between 100.degree.-250.degree. C. In order to make a more rugged tube, capable of operating with good electrical stability at temperatures in excess of 150.degree. C., a ceramic-metal envelope having a sapphire faceplate has been designed. In order to construct such a tube at reasonable cost, a plano-plano sapphire window is used rather than the plano-concave window used in the glass envelope tube of the copending patent application. A ceramic-metal tube with such a plano-plano input window and an apertured shield cup identical to that described in the copending patent application demonstrated poor electron collection efficiency and pulse height resolution.
Electron collection efficiency is defined as the ratio of the number of photoelectrons incident on the first dynode of the electron multiplier to the number of photoelectrons emitted from the photocathode. In many scintillation counting applications, such as oil well logging, a photomultiplier tube is coupled to a thallium-activated sodium iodide crystal in which scintillations are produced by gamma rays resulting from nuclear disintegrations. Because the output of a photomultiplier is linear with light input and because the light energy of scintillations is directly proportional to the gamma-ray energy over a certain range, an electrical pulse is obtained which is a direct measure of the gamma-ray energy. Consequently, an important requirement of photomultipliers used in nuclear spectrometry is the ability to discriminate between pulses of various heights. The parameter indicating the ability of a tube to perform this discrimination is called pulse-height resolution. Pulse-height resolution is defined as the width of the photopeak at half the maximum count rate divided by the pulse height at maximum count rate. Consequently, the lower the pulse-height resolution, the greater the ability of the photomultiplier to discriminate between pulses of nearly equal height.
A focusing electrode in addition to the above-described apertured shield cup is required between the photocathode and the electron multiplier in order to focus substantially all the photoelectrons from the photocathode onto the first dynode of the electron multipliers thereby improving the electron collection efficiency and the pulse-height resolution of the tube.